An attenuator is used in a transceiver or a radio frequency (RF) transmitter to attenuate power of an RF signal that is to be transmitted to a receiver. Attenuation is controlled using control voltages supplied to the attenuator. The attenuator, typically, provides non-linear output in decibels that affects performance of the receiver while receiving RF signals of low power.
An exemplary circuit 100 for generating the control voltages is illustrated in FIG. 1(A) (Prior Art). The circuit 100 includes a digital-to-analog converter (DAC) 105, an isolation circuit 110A, an isolation circuit 110B, a resistor 115A, and a resistor 115B. The DAC 105 provides two currents that are differential to each other. The control voltages are obtained at nodes 120A and 120B by enabling flow of a first current through the resistor 115A and enabling flow of a second current through the resistor 115B. A first voltage (Vs), obtained at node 110A, is linear with respect to the first current and a second voltage (Vp), obtained at node 110B, is linear with respect to the second current. The control voltages, when applied to the attenuator, generate non-linear attenuation characteristics in decibels (dB). Equation (1) illustrates the attenuation (A) of the attenuator.
                    A        =                  1                      1            +                                          (                                  a                  -                                      a                    ′                                                  )                                            k                ⁡                                  (                                      1                    -                    a                    -                                          a                      ′                                                        )                                                                                        (        1        )            where a is a ratio of the first current and a maximum current from the DAC 105, a′ is a ratio of threshold voltages of transistors used in the attenuator to which the first voltage (Vs) and the second voltage (Vp) are applied, and a product of the maximum current from the DAC 105 and resistance of the resistor 115A, and k is a constant representing a ratio of sizes of transistors of the attenuator. By substituting values of a, a′ and k in equation (1), a waveform 125 as illustrated in FIG. 1(B) can be obtained between the attenuation (A) and the first current (I). The waveform 125 indicates non-linear characteristics in dB. Further, the attenuation obtained using the control voltages is process-voltage-temperature (PVT) dependent as a′ is a function of the threshold voltages. Resolution of the DAC 105 also needs to be higher due to the non-linear attenuation characteristics in dB and dependence of the attenuation characteristics on the PVT.